1. Field of the Invention
This invention relates to variable field strength magnets and, more particularly, to multipole variable permanent magnets.
2. Prior Art
A number of techniques are available for producing variable-strength magnetic fields. Such fields are particularly useful in charged particle acclerators for bending and focusing of particle beams. Electromagnets, that is, devices which produce magnetic fields using electrical currents passing through ordinary or superconducting windings, have serious limitations for certain applications. One limitation is the large amounts of expensive electrical power that these systems consume either for the current to operate a conventional conductor or for cooling a superconductor. In addition, conventional electromagnets are limited to certain minimum volumes because their current densities are inversely proportional to their linear dimensions, which leads ultimately to insurmountable cooling problems. The result is that the currents for these electromagnets must be reduced for smaller sizes with consequently smaller magnetic fields.
And so it has been found that for many magnet applications it is often advantageous to use permanent magnets instead of electromagnets in order to eliminate windings with their consequent power consumption and to produce strong fields in physically small spaces. For magnets which are used in small spaces and which require large pole tip fields, it is very often difficult to provide enough copper cross-sectional area in the space available. An area where high-field permanent magnets find particular application is in the construction of small quadrupole magnets for guiding, focusing, and turning charged particle beams in linear accelerators used in atomic physics and medical treatment and research. A theoretical analysis is presented by J. B. Blewett in "Design of Quadrupoles and Dipoles Using Permanent Magnet Rings," Brookhaven National Laboratory Report No. AADD-89, Aug. 10, 1965. That report includes equations and analyses for maximizing the strength of a ring or cylindrical quadrupole permanent magnet using anisotropic material.
A technique for designing permanent magnet multipole magnets was disclosed in a paper by the present inventor, K. Halbach, "Design of Permanent Magnet Multipole Magnets with Oriented Rare Earth Cobalt Materials," Nuclear Instruments and Methods 169 (1980) pp 1-10. Disclosed therein is a quadrupole design which uses a number of magnetically anisotropic magnet segments, each having an easy axis, or axis of magnetic orientation, in a different predetermined direction. One proposed application of this design combines two multipole magnets such that one quadrupole is located within the aperture of the other. For the rare earth cobalt (REC) materials used, superposition of the individual magnetic fields is possible, and the fields of each quadrupole add or subtract depending upon their relative rotational positions. This design suffers from fringe fields at the ends of the magnet which combine to produce undesired perturbations in the beam optical properties of the magnet.